Circulation and the heart Flashcards
Outline – Circulatory System
- One of the most important ensuring nutrient delivery to tissues for energy, and waste removal for homeostasis
- Electrical signals within the heart – how is it coordinated, how does it lead to heart pumping, what cells are responsible for this
- Blood = Transport medium for body’s “materials”
- Blood Vessels = Blood passageways to and from heart and tissues.
- Heart = Pump that establishes the pressure gradient for blood flow
two types of Cardiac Muscle Cells
- Myocardial contractile cells
- Myocardial autorhythmic cells
–> only containing few contractile fibres and no organised sarcomere
end to end junctions form between these cells = call them intercalated disks
intercalated disks :
1. contain gap junctions == channels that allow ions to flow through in order to propagate those action potentials that are generated
2. desmosomes == transmembrane proteins which tether the contractile cells to one another
–> so that as you as the muscles start to generate force, that force is then conducted throughout the entire heart tissue.
how does Myocardial Autorhythmic Cells
this is due to what’s known as the pacemaker potential.
Normally, the inside of a resting cell is negatively charged compared to the outside. For these pacemaker cells, after they fire off an electrical impulse, they automatically start to become less negative, slowly moving towards a positive charge.
ion channels known as funny channels
–> remain open at around -60mV
1. the influx of sodium ions = depolarisation
2. resting membrane potential becomes more positive and ion funny channels close up/ Ca2+ channels open
3. at its peak, Ca2+ channels close and K channels opn
4. during repolarisation = K+ channels close
5. at -60mV, Ion funny channels open again and cycle repeats
Electrical Activity of the Heart
Sinoatrial node (SA node)
* Specialised region in right atrial wall near opening of superior vena cava
* Pacemaker of the heart
Atrioventricular node (AV node)
* Small bundle of specialised cardiac cells located at base of right atrium near septum
Bundle of His (atrioventricular (AV) bundle)
* Cells originate at AV node and enters interventricular septum
* Divides to form right and left bundle branches that travel down septum, curve around tip of ventricles, travel back toward atria along outer walls
Purkinje fibers
* Small, terminal fibers that extend from bundle of His and spread throughout ventricular myocardium
Electrical Signals Coordinate Contraction in the heart
- SA node depolarises
- Electrical activity goes rapidly to AV node via internodal pathways
- Depolarisation spreads more slowly across atria. Conduction slows through AV node
- Depolarisation moves rapidly through ventricular conducting system to the bottom of the heart
- Depolarisation wave spreads upward from the apex
Electrocardiogram (ECG)
Extracellular recording of the sum of Aps occurring in multiple heart muscle cells
1. atrial depolarisation (small bump)
2. ventricle depolarisaion (large peak)
3. ventricle repolarisation (moderate bump)
—> atrial repolarisation happens silmutaneouly with ventricle depolarisation (QRS complex) so not seen on ECG
Two major components: Waves & Segments
1. P wave : atrial depolarisaation
2. P-Q or P-R segment : atria contract
3. Q wave : atrial repolarisation
4. R wave : spreading through septum
5. S wave : depolarisation of ventricles
6. S-T segment : ventricles contract
7. T-wave : ventricular repolarisation
QRS complex = ventricular depolarisation and is a combination of the Q wave, R wave and S wave
Myocardial Contractile Cells
AP generated are similar to neurons and skeletal muscle cells
* Na + influx causes depolarisation
* K+ efflux causes repolarisation
Main difference is the influx of Ca2+
* Lengthens the AP
- Na+ channels
- Na+ channels close, fast K+ channels open
- Ca2+ channels open, fast K+ close
- Ca2+ channels close, slow K+ channels open
- Resting potential
Why is this lengthening important in cardiac muscle contraction?
cardiac muscle fiber : refractory period almost as long as the entire muscle twitch
long refractory period in a cardiac muscle prevents tetanus
mechanical cardiac cycle
- late diastole
every chamber of the heart is relaxed
passive flow of blood into atrium and passively fills into ventricles too - atrial systole
SA node fires, atrial depolarisation, blood squeezed into ventricles and atriums are emptied
AV node delay, passes depolarisation signal through septum into the myocardium and causes the ventricles to begin contracting - isovolumic ventricular contraction
blood starts to move upwards and AV valve closes, and semi lunar valves closed.
no change in volume, a significant amount of pressure within ventricles
pressure reaches a certain threshold of pressure - ventricular ejection
pressure opens semi lunar valves to open
blood ejectec out and travels through pulmonary semilunar valves again to your lungs - isovolumic ventricular relaxation
ventricles relax with blood ejected
pressure in ventricles fall
blood flows back into cusps of semiluar valves and snaps them closed
pressure volume cycle
- volume increases but pressure remains the same
- A’ when atrial depolarisation occurs, and starts to contract blood from atrium into ventricles so volume increases a little
- isovolumetric ventricular contraction : AV valves close, volume remains the same, but pressure increases steadily,
- ventricular contraction: high pressure causes semi lunar valves to open blood is ejected and volume decreases
- volume returns back to normal but pressure decreases signfiantly
heart sounds
closing of valves
1. first heart sound : closing of AV valve during inital stage of isovolumetric ventricular contraction
2. second heart sound : closing of semilunar valves during isovolumetric ventricular relaxation
laminar flow is when blood is flowing in one direction = no sound
turbulent flow is when blood flow is disturbed when the valve shuts = makes sound
